The present invention relates to a circuit board, a method for fabricating the circuit board and an electronic device, more specifically a circuit board for electronic parts, such as semiconductor elements, etc., to be mounted on, a method for fabricating the circuit board and an electronic device including the circuit board.
As electronic equipments typically represented by portable terminal devices, for example, are increasingly downsized and have higher performance, it is strongly required that electronic elements (e.g., semiconductor elements) to be mounted on the electronic equipments and circuit boards for the electronic elements to be mounted on are increasingly downsized and thinned, and have higher performance and higher reliability.
In order to meet the requirements, the so-called bear chip mounting which mounts directly electronic elements on circuit boards has been increasingly widely used as a mounting method for mounting the electronic elements on the circuit boards. As the electronic elements have more pins, the circuit boards for the electronic elements to be mounted on must have higher density, and the circuit boards have accordingly multi-layers.
Multi-layer circuit boards (the so-called build-up boards) formed by the build-up technology which insulating layers and conducting layers are alternately layered on one surface or both surfaces of core materials by thin film forming techniques are practically used. As the core materials of the build-up boards, organic materials, such as glass epoxy resin, etc. are generally used.
A build-up board, which has a layer structure of insulating layers and conducting layers laid by a thin film forming technique, can have micronized patterns. Accordingly, bear chips can be mounted directly on the build-up board (bear chip mounting). However, when the bare semiconductor chips are mounted on a build-up boards using a conventional organic material as a core material, the silicon chips having a thermal expansion coefficient of about 3.5 ppm/° C. are mounted directly on the build-up board having a thermal expansion coefficient of a 10–20 ppm/° C. Even with an under-fill provided, thermal stress is generated in the connection due to the thermal expansion difference between the two, and the connection reliability is decreased.
In order to mitigate such stress, a method of lowering an elastic modulus of an adhesive as an under-fill, etc. are practically used. However, as chip sizes become larger, it is apparent that even such method will not be able to sufficiently ensure the reliability of the connection. In order to ensure high reliability of the connection to the build-up board it is essential to lower the thermal expansion coefficient of the build-up board itself.
In order to reduce noises of devices for higher performance, generally decoupling capacitors are connected to the chips. In this case, depending on some mounting technologies, capacitors are often disposed on a side of the board, which is different from the side where the chips are mounted. In this case, it is preferable to make the circuit board as thin as possible from the viewpoint of shortening the connection distance between the chips and the capacitors so as to reduce the inductance.
In such background, conventionally metals or ceramics whose thermal expansion coefficients are smaller than the organic core materials have been used. The metal core boards can be fabricated by forming holes for through-holes in the core metal substrate, building up a prepregs and copper foils sequentially on both surfaces of the metal core substrate, forming the through-holes from the outer layer through the holes formed in the metal core substrate, plating copper non-electrolytically and electrolytically to form a circuit pattern on the outer layer. Specific materials of the core material are generally, as the metals, aluminum, copper, silicon steel, nickel-iron alloy, CIC (copper/invar/copper clad material) and aluminum nitride as the ceramics.
Of these materials, aluminum, etc. are light, but the thermal expansion coefficients of them are larger than that of silicon unpreferably in terms of the connection reliability. On the other hand, the thermal expansion coefficients of invar, covar, alloys, such as silicon steel, and a clad material, such as CIC, are substantially the same as the thermal expansion coefficient of silicon. However, they have large specific gravities and add weights unsuitably to be used in the circuit boards, which are processed with the large-sized cores included. Their Young's moduli of elasticity are not high, and large core substrate undesirably have bowing and waves, which causes troubles in the build-up process and in mounting semiconductor elements.
It is difficult to form thin substrates of refractory metals, such as molybdenum and tungsten whose thermal expansion coefficients are relatively approximate to the thermal expansion coefficient of silicon and which have large specific gravities. Large substrates of them are also heavy for easy handling. As for ceramics, aluminum nitride, etc., whose thermal expansion coefficients are near the thermal expansion coefficient of silicon, is very difficult to form through-holes and vias. The ceramic core substrate must be formed by cofire, which makes it impossible to provide large boards and adds to costs.